It will readily be understood that it is absolutely essential from the safety point of view for such eyeholes and armholes to be closed in an absolutely pressure-tight manner when the steam generator is in operation. However, it can happen that dirt in the threads of the tapped holes, in particular due to work that produces dust and debris in the vicinity of such eyeholes or armholes, prevents the covers of such orifices from being bolted into place properly, and this can give rise to safety problems when the steam generator is put back into operation.
It is therefore necessary, in particular after work has been performed, to clean very carefully all of the tapped holes used for bolting on covers.
In the past this operation has been performed manually. However, since the task is tedious (in particular because of the large number of tapped holes that need cleaning), it is often not performed completely reliably.